Method and means for minimizing reflec-



METHOD AND MEANS FOR MINIMIZING REFLEC- TION OF HIGH-FREQUENCY RADIO WAVES Otto Halpern, Boston, Mass., assignor to the United States of America as represented by the Secretary of the Navy No Drawing. Application March 5, 1945 Serial No. 581,179

13 Claims. (Cl. 343-18) This invention relates broadly to a method and means for minimizing the reflection of radio waves of preselected wavelength in the very high-frequency range of approximately 20 to 100,000 megacycles, incident upon surfaces and objects which normally would reflect such waves. The invention is more specifically related to a method and means for minimizing or modifying the radio wave reflecting characteristics of objects against searching radio-echo detection apparatus and the like.

It is known in the art how to protect good reflectors like electrically conducting surfaces from incident radio waves of high frequencies. This may be done in two different ways, which may be conveniently designated as the non-resonant and the resonant methods, respectively.

In the case of the known non-resonant method the reflecting surface is covered by a substance of small reflecting power and small high-frequency loss, but of a thickness sufficient so that waves penetrating through to the under surface and being there reflected, are sufii ciently weakened to be negligible when re-cmerging. Mathematical analysis of that problem shows that the thickness of such layers must be so great that the method is of very limited practical value.

The second known or resonant method consists in affixing a sufliciently high-loss dielectric material layer either directly to the metallic surface of the reflector which is to be protected, or to a metallic sheet which, in turn, may itself be so applied. In this case the dielectric material must have a thickness which is an odd multiple of one-quarter of the wavelength of the incident radiation, measured inside the material, and a high-frequency loss of suflicient magnitude which may be due to either surface treatment of the material or internal losses in it. This method has been used principally for laboratory (and demonstration) purposes, since the resonant thickness for frequencies even as high as 1,000 mega-cycles is five (5) centimeters, or thereabout.

The present invention is an improvement upon both of said prior-known methods and makes them practicable for use in a variety of applications. The improvement is achieved by the provision of a layer preferably comprising myriads of electrically conducting particles dispersed in an appropriate binder, the particles being quasiinsulated each from the other.

Electrically conducting particles thus arranged will impart to the whole medium or layer a dielectric constant which will be greatly in excess of that of the binder alone. It can be made to assume very large values. The physical reason for this fact can best be understood by the analogy of gas atoms dispersed and insulated from each other by intervening free space. Under the influence of an electric field, such gas atoms will assume an electric moment which will be the higher, the smaller the depolarizing factor of the individual particles, and, also, the higher the greater the concentration thereof.

The depolarizing factor of an individual particle or atom is known to depend on its shape and on its orienta- United States Patent 0 2,923,934 Patented Feb. 2, 1960 tion with respect to an electrical vector influencing it. The factor is large in the case of spherical or spheroidal particles, and small for thin threads or flakes, if the thread, or the plane of large dimension of the flake, is oriented in the direction of the electrical vector. The theory with respect to concentration constitutes so difficult a problem that no reliable quantitative estimates can be made.

The artificial dielectric constant imparted by the metallic particles also has an imaginary part which is equivalent to the existence of a loss in the dielectric medium. The actual high-frequency value of the dielectric constant varies somewhat with frequency.

To obtain a high value of the dielectric constant, it is therefore advisable to employ flake particles in dense concentration dispersed and so oriented in the binder that the plane of the large dimension of the flakes is parallel to the plane of the aggregate. In the case of threads, a similar result may be obtained by dispersing and orienting the same in the direction of the electrical vector and parallel thereto.

While every type of electrically conducting particle gives rise to an artificial dielectric constant only ferromagnetic particles can be used to increase the magnetic permeability.

The determining factors for increasing permeability or susceptibility are substantially the same as in the case of the dielectric constant. The characteristic parameter is the demagnetizing factor which is identical with the depolarizing factor. It is, therefore, possible to impart to a medium comprising myriads of metallic particles dispersed in a binder, a magnetic susceptibility which will be large if the concentration of the particles is high, and the demagnetizing factor is small. This latter characteristic can be readily achieved by using flakes or threads of ferromagnetic materials dispersed and oriented with respect to the magnetic vector of the incident wave.

In practice layers employing metallic flakes rather than threads have been found advantageous. Such flakes are available on the market for other purposes.

Conductors like aluminum, copper, iron, steel, Permalloy and graphite flakes and mixtures thereof, have been utilized in accordance with the invention. The flakes average in thickness between 3 10- and 2X 10- centimeters, with the long dimension as high as seventy times the average thickness. The commercial processes for producing so-called bronze paint pigment of various metals yield flakes which are satisfactory in size and shape for the purposes of this invention. The shapes vary from that of disks to that of thread-like particles.

The grease present on the surfaces of commercial flakes depends upon the production process and can be varied so as to permit ready and optimum orientation. The flakes may be dispersed and thoroughly mixed in a variety of binder substances and materials such as waxes, resins, polystyrene, Vistanex, Vinylite or synthetic rubbers. Xylene and toluene are examples of suitable volatile solvents. Practically useful concentrations vary between 20 and percent of metal content of the mixture.

The composition or mixture of flakes, binder and solvent may be applied by spraying or working with a flat smoothing tool at room temperature or slightly higher. Applied in fluid form the composition readily dries and becomes fixed as a layer and coating, as the solvent evaporates.

It is well known that with a composition prepared and applied in the above-described manner, the flakes generally assume the preferred orientation, i.e., parallel to the surface covered. Working the coating by a smoothing or stroking motion aids in obtaining a more uniform leafing of the flakes with their faces generally parallel to the plane of the layer. Although the resulting layer may be obtained in a single coating, it has been found that a composite layer constructed of a plurality of thin coatings is more eflicient. Multiple coatings as just described tend to yield an effective dielectric constant which is higher than that obtained with a single coating application, because a better leafing of the flakes is obtained when the coatings are thin and individually worked.

When employing the non-symmetrically commercially obtained flake particles the layer may be made to be polarized" to a certain degree, by causing the smoothing stroke to be always in the same direction. That mode of application and treatment tends to align and orient the long dimension of the flakes as well as to press the faces of the flakes parallel to the plane of the layer. By altering the direction of spraying or the smoothing stroke ninety (90) degrees in the successively applied coatings a layer may be made which is isotropic in the plane thereof.

It may be expedient in some instances to coat the nonreflecting layer onto a thin fabric or on a metal foil which in turn may be simply interposed or applied to the surface to be rendered non-reflective.

If required, the composite dielectric layer may be formed first, and later applied to a surface with an adhesive. In the case of resonant layers the thickness and electromagnetic influence of the adhesive by which the layers herein described may be attached to a supporting metal foil or to the object itself must be taken into account. If the magnetic permeability of the dielectric is unity 1), and if as is usually the case the thickness of the adhesive is small compared to one-quarter the wavelength of the radiation, measured inside the material, then the simple rule obtains that the total thickness of dielectric and adhesive together must be an odd multiple of one-quarter the wavelength, measured inside the dielectric. If the magnetic permeability of the dielectric is greater than unity (l), the required computations are known by those skilled in the art.

A number of representative layers made with different binders, particles and concentrations and with varying method of preparation in accordance with the invention will now be described to illustrate the general explanations given hereinabove and to make clear the theoretical aspects necessary to attain high dielectric constants. The frequencies used in the measurements referred to are in the neighborhood of 2,500 megacycles or higher values.

Example N0. 1.Aluminum flake 76%, Vistanex and Estergum 24%, prepared by cross smoothing. Index of refraction 57.5. Reflected intensity at resonance 8%. No directional properties in plane of film.

Example N0. 2.-Aluminum flake, polystyrene with plasticizer (70%, 15% and 15%) by cross smoothing. Index of refraction 45. Intensity reflected at resonance 1%. No directional properties in plane of film.

Example No. 3.Copper flake in polystyrene, 37 /2 and 62 /2% pressed. Index of refraction 6. Intensity reflected at resonance immeasurably small. No directional properties in plane of film.

Example No. 4.-Aluminum flake in polystyrene, 37V: and 62%% pressed. Index of refraction 8.8. Intensity reflected at resonance immeasurably small. No directional properties in plane of film.

Example No. 5.-Steel flake, plus clay, plus synthetic rubber binder known as GR-I, 40%, 30%, 30%. Calendered and pressed, one layer crossed on top. Index of refraction 7.9. Intensity reflected at resonance 1%. No directional properties in plane of film.

Example N0. 6.Aluminum flake in Vistanex and Estergum, 80%, 15%, 5%. Hand smoothed. Principal indices of refraction 64.5 and 71.5. Intensities reflected at resonance 22% and 16%.

Example N0. 7.-Aluminum powder in synthetic rubber binder known as GR-S, 75% and 25%! Han mixed, hand smoothed. Principal indices of refraction 46 and 52. Intensities reflected at minimum 31% and 3%.

The uses to which such materials and compositions may be put are very diversified. Layers of high dielectric constant, in accordance with the invention are relatively thin as compared to the thick protective layers required by the prior art. Thus, resonant method protection layers of relative thinness are made available for the first time.

Resonant absorption of incident waves by protective layers is obtained as is well known, if the thickness of the layer is equal to an odd multiple of thickness of one-quarter A) wavelength the index of absorption should be closely The index of refraction and the index of absorption are respectively the real and the imaginary parts of the square root of the generally complex dielectric constant.

The use of a material of high dielectric constant as set forth in the above table of representative sample films, is, therefore, capable of reducing the resonant thickness by a factor in the neighborhood of fifty (50).

In some applications of resonant layers, in accordance with the invention, it is advantageous to purposefully reduce the value of the dielectric constant, or, in other words, to choose an appropriate dielectric of not too high a dielectric constant for either of two possible reasons: (a) the thickness of the indicated layer is so small as to be mechanically difiicult to attain; or, (b) the resonance curve is too sharp for the application under consideration. Theory shows that in most cases the width of the resonance curve is proportional to the square root of the ratio of the real parts of the permeability and the dielectric constant.

The particular problem of protection presented determines whether a large or a small bandwidth is required and also the choice of the material and its dielectric constant. Lowering of the dielectric constant from its maximum obtainable values can be achieved in a wide variety of ways, for example, by reducing the particle content, reducing the degree of orientation of the particles, or lowering the ratio of length to thickness of the flakes by selecting flakes of appropriate dimensions.

It is well known that any substance will become a selective reflector if its thickness is made one-half /2) the wavelength, measured inside. Theory also shows such layers become more selective the larger their dielectric constant becomes. Thus, the use of the herein described materials otfers a practical way to protect an object from reflection of all radio wavelengths within a reasonable range, except those of a narrow selective band.

The thickness of resonant layers can be further reduced in accordance with the invention, by the use of particles having a permeability greater than unit (1). Thus, not only is the advantage of the artificial dielectric constant attained, but, at the same time, the index of refraction of such layer becomes equal to the square root of the product of the permeability and the dielectric constant, while the resonant value of the index of absorption has to be approximately equal to TI times the real value of the magnetic permeability. The bandwidth may also be increased, as will be understood om the explanation given hereinbefore.

known techniques.

The advantage of using particles having a permeability greater than unity 1) likewise applies to non-resonant layers. Theoretically, reflection from a layer which is as thick as or thicker than the skin depth becomes small if the real as well as the imaginary part of the dielectric constant and the magnetic permeability approach equality. The skin depth is defined as the thickness in which the amplitude of the incident wave decreases to of its value.

If the material is so selected as to show different values of dielectric constant and permeability in the plane of application (by smoothing or spraying, or by the use of threads), it is possible to construct a resonant layer which is a good absorber for one manner of polarization of the incident waves, while it is a good reflector thereof for the other manner of polarization. This is due to the fact that the resonant thicknesses vary in value for the two principal axes in the plane of the material, being determined by the respective values of the dielectric constant in those two directions. The problem under consideration will determine whether a material which is isotropic or anisotropic in its plane, should be used. If thread particles (as distinguished from flakes) are employed, a layer which is not cross-smoothed or crosssprayed or cross-laminated will be anisotropic.

Thus, appropriate layers can be used to protect objects against radiation of a certain preselected wavelength or against radiation of a certain preselected wavelength band. Such layers may be used on exterior objects or in the inner parts of radar apparatus, for example, in terminations, and applications may be found useful in all cases where unwanted radio echoes occur due to surrounding objects.

For practical purposes the proper layer thickness may be determined by first measuring the real and imaginary parts of the dielectric constant and likewise of the magnetic permeability. This may be done in accordance with It has been found that for waves of a frequency of 3,000 megacycles the thickness of the resonant layer (quarter-wavelength) may be as thin as about one-half millimeter. If the bandwidth of the layer is to be increased, this dimension must be increased, as previously described.

The real part of the high-frequency dielectric constant of layer materials can be made to range from about 20 or lower to over 6,000. The main factors which determine the value are the thickness to length ratio of the particles and the care taken in aligning the particles in the plane of the layer. The latter depends somewhat on the grease content on the surface of the flakes which is added in the production thereof. The imaginary part may range to higher than 500, depending on the parameters involved. The relative values of the real and imaginary parts may be adjusted for maximum absorption. That may be varied by selecting different relations between volume concentration of the flakes, resistivity of the flake material, and also the flake alignment.

The foregoing description has been limited to cases of reflection at normal incidence, but the invention is equally applicable to cases of reflection at oblique incidence as those skilled in the art will recognize.

The invention includes the use of the materials described for other purposes than those hereinbefore mentioned, as, for instance, devices to change the propagation of waves especially inside wave guides and coaxial lines, high capacity condensers of small size having high dielectric constant materials, high permeability materials for radio-frequency inductances and the like. The metallic particles can be oriented so as to be aligned with the electric or magnetic fields to be imposed. Casting or molding techniques may be employed to obtain desired 6 shapes of the objects to be formed and alignment of the particles, flakes or threads may be accomplished by the application of high electric or magnetic fields depending on the material of the particles.

Many other uses of the invention will be obvious to those skilled in the art and need not be described herein. And while there has been described herein certain preferred embodiments of the invention and manners of practicing the methods thereof, it will be apparent, likewise to those skilled in the art, that various changes and modifications may be made without departing from the invention, and it is therefore intended that the appended claims shall cover all such changes and modifications as fall within the gist and scope of the invention.

I claim:

1. The method of selectively minimizing the reflection of radio waves of preselected wavelength in the very high-frequency range of approximately 20 to 100,000 megacycles, incident upon metal surfaces normally reflecting such waves, which comprises forming on the said surface a coating-layer of myriads of finely-divided electrically conducting particles dispersed in a binder of insulating-material, said layer having an index of absorption of substantially 2/11- times the real part of the magnetic permeability of said layer at said preselected wave length, the length of each of said particles being substantially greater than the thickness thereof, orienting said particles with the thickness dimensions thereof in substan tial parallel relation in said layer, and predeterminedly limiting the thickness of the layer to an odd multiple of M412 where )t is said preselected wavelength and n denotes the index of refraction of said coating layer.

2. The method of selectively minimizing the reflection of radio waves of preselected wavelength in the very high-frequency range of approximately 20 to 100,000 megacycles, incident upon metal surfaces normally re flecting such waves, which comprises forming on the said surface a coating-layer of myriads of finely-divided electrically conducting particles dispersed in a binder of insulating material, said layer having an index of absorption of substantially 2/11 times the real part of the magnetic permeability of said layer at said preselected wave length, the length of each of said particles being substantially greater than the thickness thereof, orienting said particles with both the length and the thickness dimensions thereof respectively, in substantial parallel relation in said layer, and predeterminedly limiting the thickness of said layer to an odd multiple of \/4n where A is said preselected wavelength and n denotes the index of refraction of said coating layer.

3. The method of selectively minimizing the reflection of radio waves of preselected wavelength in the very high-frequency range of approximately 20 to 100,000 megacycles, incident upon metal surfaces normally reflecting such waves, which comprises forming on the said surface a coating-layer of myriads of finely-divided electrically conducting thin flake particles dispersed in a binder of insulating material, said layer having an index of absorption of substantially 2/ 1r times the real part of the magnetic permeability of said layer at said preselected wave length, the long dimension of each of said flake particles being substantially greater than the thickness thereof, orienting said flake particles with the thickness dimensions thereof in substantial parallel relation in said layer, and predeterminedly limiting the thickness of the said layer to an odd multiple of M41: where A is said preselected wavelength and n denotes the index of refraction of said coating layer.

4. A coating-layer adapted to minimize the reflection of radio waves of preselected wavelength in the very highfrequency range of approximately 20 to 100,000 megacycles, incident upon metal surfaces normally reflecting such waves, comprising myriads of finely-divided electrically conducting particles dispersed in a binder of insulating material, said layer having an index of absorption of substantially 2/1r times the real part of the magnetic permeability of said layer at said preselected wave length, the length of each of said particles being substantially greater than the thickness thereof, said particles being oriented in the layer with the thickness dimensions thereof in substantial parallel relation, said layer, including the dispersed particles and binder, having a predetermined thickness equal to an odd multiple of M41: where k is said preselected wavelength and n denotes the index of refraction of said coating layer.

5. A coating-layer adapted to minimize the reflection of radio waves of preselected wavelength in the very highfrequency range of approximately 20 to 100,000 megacycles, incident upon metal surfaces normally reflecting such waves, comprising myriads of finely-divided electrically conducting particles dispersed in a binder of insulating material, said layer having an index of absorption of substantially 2/1r times the real part of the magnetic permeability of said layer at said preselected wave length, the length of each of said particles being substantially greater than the thickness thereof, said particles being oriented in the layer with the length and the thickness dimensions thereof, respectively, in substantial parallel relation, said layer, including the dispersed particles and binder, having a predetermined thickness equal to an odd multiple of \/4n where A is said preselected wavelength and n denotes the index of refraction of said coating layer.

6. A coating-layer adapted to minimize the reflection of radio waves of preselected wavelength in the very highfrequency range of approximately 20 to 100,000 megacycles, incident upon metal surfaces normally reflecting such waves, comprising myriads of finely-divided electrically conducting flake particles dispersed in a binder of insulating material, said layer having an index of absorption of substantially 2/1r times the real part of the magnetic permeability of said layer at said preselected wave length, the length of each of said flake particles being substantially greater than the thickness thereof, said flake particles being oriented in the layer with the thickness dimensions thereof in substantial parallel relation, said layer, including the dispersed particles and binder, having a predetermined thickness equal to an odd multiple of M41: where A is said preselected wavelength and n denotes the index of refraction of said coating layer.

7. A coating-layer adapted to minimize the reflection of radio waves of preselected wavelength in the very highfrequency range of approximately 20 to 100,000 megacycles, incident upon metal surfaces normally reflecting such waves, comprising myriads of finely-divided electrically conducting flake particles dispersed in a binder of insulating mateiral, said layer having an index of absorption of substantially 2/1r times the real part of the magnetic permeability of said layer at said preselected wave length, the length of each of said flake particles being substantially greater than the thickness thereof, said flake particles being oriented in the layer With the length and the thickness dimensions thereof, respectively, in substantial parallel relation, said layer, including the dispersed particles and binder, having a predetermined thickness equal to an odd multiple of A/4n where A is said preselected wavelength and n denotes the index of refraction of said coating layer.

8. A solid dielectric having a predetermined dielectric constant, the real part of which within the range of approximately 20 to 6,000 at the high radio frequencies of approximately 20 to 100,000 megacycles, said dielectric comprising a composite layer of myriads of finelydivided electrically conducting particles the length of each of which is substantially greater than the least dimension thereof, dispersed in a binder of insulating material, said particles being oriented in the layer with the least dimensions thereof in substantial parallel relation, the dielectric constant of the binder at the said high frequencies being substantially less than that of said constant of the layer.

9. A solid dielectric having a predetermined dielectric constant, the real part of which within the range of approximately 20 to 6,000 at the high radio frequencies of approximately 20 to 100,000 megacycles, said dielectric, comprising a composite layer of myriads of finelydivided ferromagnetic metallic flake particles dispersed in a binder of insulating material, said flakes being oriented in said binder with the thickness dimensions thereof in substantial parallel relation in the layer, the dielectric constant and the magnetic susceptibility of the binder at the said high frequencies being substantially less than that of the said constants of the layer.

10. A solid dielectric layer adapted selectively to transmit radio waves within a selected narrow band of wavelengths situated within the high radio frequency range of approximately 20 to 100,000 megacycles, said layer having a dielectric constant within the range of approximately 20 to 6,000, said layer consisting of myriads of finely divided metal flake particles, the length of each of which is substantially greater than the least dimension thereof, dispersed in a binder of insulating material, said particles being oriented in said layer with the least dimensions thereof in substantially parallel relation, the thickness of said layer being substantially one-half of a preselected wave length, measured inside the layer, of a wave length within said band.

11. A solid dielectric layer adapted to minimize reflections of radio waves incident on metal surfaces normally reflecting such waves, said layer having a dielectric constant the real part of which is within the range of approximately 20 to 6,000 at the high radio frequencies of approximately 20 to 100,000 megacycles, said dielectric comprising a composite layer of myriads of finely divided ferro-magnetic particles, the length of each of which is substantially greater than the least dimension thereof, dispersed in a binder of insulating material, said particles being oriented in the layer with the least dimensions thereof in substantially parallel relation, the thickness of said layer being greater than skin-depth, and the dielectric constant and the magnetic susceptibility of the binder at said high frequencies being substantially less than said constants respectively of the layer.

12. A solid dielectric layer adapted to minimize reflections of radio waves throughout a preselected wave length band in the very high frequency range of approximately 20 to 100,000 megacycles, incident on surfaces normally strongly reflecting such waves, said layer comprising myriads of finely divided electrically conducting ferro-magnetic particles dispersed in a binder of insulating material, the dielectric properties of said binder and the electrical and magnetic properties of said ferromagnetic particles and the concentration thereof in said binder being so chosen that the square root of the resulting complex dielectric constant of said layer is substantially equal to the square root of the resulting complex permeability of said layer, and the thickness of said layer being substantially in excess of skin depth.

13. The method of selectively permitting transmission through a dielectric layer, of radio waves within a preselected narrow band of wavelengths situated within the high radio frequency range of approximately 20 to 100,000 megacycles, incident on surfaces normally strongly reflecting such waves, which comprises: applying to said surface a coating layer having a dielectric constant within the range of approximately 20 to 6,000, and consisting of myriads of finely divided metal flake particles, the length of each of which is substantially greater than the least dimension thereof, dispersed in a binder of insulating material, orienting said particles with the thickness dimensions thereof in substantially parallel relation in said layer, and adjusting the thickness of said layer to substantially one-half of the wave length, meas' ured inside the layer, of a wavelength within said narrow 2,293,839

band. 2,391,376

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2,235,489 Rath Mar. 18, 1941 ,7

2,247,036 Ruben June 24, 1941 802,728

'10 Linder Aug. 25, 1942 Monack Dec. 18, 1945 Pratt et a1. Apr. 8, 1947 Korn et a1 Feb. 24, 1948 FOREIGN PATENTS Great Britain Feb. 11, 1935 Australia May 21, 1935 France Sept. 14, 1936 

